1. Field of the Invention
The invention relates to apparatus, and an accompanying method for use therein, for use in a conventional dynamic thermo-mechanical material testing system in order to advantageously provide enhanced self-resistive specimen heating that yields greater temperature uniformity throughout a specimen under test than heretofore achieved.
Advantageously, our invention also finds use in a conventional dynamic mechanical material testing system in order to self-resistively heat a test specimen uniformly over its entire volume. Such heating can be controlled and synchronized to a mechanical test program, e.g., a predefined series of mechanical deformations, to impart a desired thermo-mechanical test program to the specimen for use in physical simulation and/or other material testing applications.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products and hence occupy an extremely important part of the world economy. As such, during manufacturing, various properties and costs of these materials need to be carefully controlled to maximize their utility and value in a given application.
Different metallic materials possess widely varying mechanical, metallurgical and other properties. Different applications necessitate use of materials with different properties. The specific properties required of a material for use in a given application are first determined followed by selection of a specific material that exhibits appropriate values of these properties.
During their initial production, metallic materials are generally formed into slabs or ingots and then from there controllably deformed into standard sized sheets, rods or coils using, e.g., conventional rolling, forging and/or extruding operations. However, correctly configuring a rolling mill, forge or extruder to properly deform production stock and impart desired physical and/or metallurgical characteristics to the material can be a tedious, time-consuming and expensive process—particularly since a production machine needs to be taken out of productive use for an extended time to properly adjust its operational parameters. Consequently, to avoid such downtime, the art teaches the general concept of determining properties of interest by testing relatively small specimens of each such material under consideration. One such technique for doing so is so-called “physical simulation”. Ideally speaking, this technique, through use of a dynamic material testing system, permits each such specimen to undergo appropriate mechanical deformation and, where appropriate, simultaneous thermal processing that, collectively speaking and to the extent possible, accurately mimic, in a small-scale environment, strains and other phenomena that the same material (but of a far larger scale) would experience through an actual production operation, such as rolling, extrusion or forging. Such simulations, when accurately done, permit proper operational parameters of corresponding production machinery to be readily ascertained and, concomitantly, minimize non-productive downtime and its associated high costs.
One crucial property of metallic materials is their ability to conduct electricity. Absent operation at superconductive temperatures, a metallic object possesses a resistance to electrical current flow proportional to its length and resistivity and inversely proportional to its cross-sectional area. Owing to its resistance, the object will generate heat whenever an electric current is passed through it. This form of heating, i.e., so-called “self-resistive heating”, finds use in a wide number of diverse applications. To the extent relevant here, dynamic material thermo-mechanical material testing systems can employ self-resistive heating to impart a desired thermal profile to each specimen prior to its being deformed in order to more accurately simulate material temperatures that will be experienced during a production operation.
Generally, in a conventional dynamic thermo-mechanical material testing system, a compressive specimen is held between two anvils or, in the case of a tensile specimen, gripped at each of its two ends in a jaw system. Since the following discussion applies equally well to both compressive and tensile testing, for simplicity, we will simply confine that discussion to compression testing.
For compression testing, the specimen is typically in the form of, for example, a small cylinder of a given material and has a substantially uniform circular cross-sectional area. Such specimens may be on the order of, e.g., 10 mm in diameter and 15 mm long; though other sizes are readily used as well. An electric current is serially passed from one anvil to another and hence generally cross-wise end-to-end through the specimen to generate a rapid, but controlled, heating rate throughout the specimen. Simultaneously therewith, various measurements are made of the specimen. Depending upon the specific measurements being made, the specimen either may or may not undergo controlled compressive deformation while it is being heated. If the specimen is to be deformed, then this deformation can be accomplished by moving one of the two anvils, at a controlled rate with respect to the other, in order to squeeze the specimen by imparting a given compressive force to the specimen. This process may be repeated several times, at differing amounts and rates of deformation, in order to impart a succession of different deformations to the specimen, thus yielding differing and accumulating amounts of strain in the specimen. Physical measurements, such as illustratively specimen dilation and temperature, are typically made while heating or cooling and deformation are simultaneously occurring. This testing not only reveals various static properties of the specimen material itself, such as its continuous heating transformation curve, but also various dynamic properties, such as illustratively hot stress vs. strain rates and hot ductility; the dynamic properties being particularly useful in quantifying the behavior of the material that will likely occur during rolling, forging, extrusion or other material forming and/or joining operations. One system that provides excellent dynamic thermo-mechanical testing is the GLEEBLE 3500 system manufactured by the Dynamic Systems, Inc. of Poestenkill, N.Y. (which also owns the registered trademark “GLEEBLE” and is the present assignee). Advantageously, this system self-resistively heats the specimen in order to generate transverse, essentially isothermal planes along the entire specimen, i.e., the specimen material in each plane uniformly heats as current passes longitudinally through that plane of the specimen. Consequently, density of the electrical heating current will be relatively uniform throughout that cross-section and, as such, will cause substantially uniform heating over that entire cross-section. Examples of such systems are described in the following United States patents, all of which are incorporated by reference herein: U.S. Pat. No. 6,422,090 (issued to H. S. Ferguson on Jul. 23, 2002); U.S. Pat. No. 5,195,378 (issued to H. S. Ferguson on Mar. 23, 1993); and U.S. Pat. No. 5,092,179 (issued to H. S. Ferguson on Mar. 3, 1992).
In such systems, the anvils need to withstand the relatively high forces imparted to the specimen without noticeably deforming themselves. Hence, these anvils are physically much larger and considerably more massive than the specimens. Consequently, for a given amount of self-resistive heating current serially passing through both the anvils and the specimen, the anvils will attain a much lower temperature than the specimen. As a result, longitudinal temperature gradients will appear end-to-end along the specimen, with a central work zone of the specimen being hottest and specimen temperature falling off towards each anvil—even though the self-resistive heating currents cause essentially isothermal planes to occur transversely across the specimen. These gradients tend to limit the work zone of the specimen to its central region, thus decreasing the effective length of the specimen for the simple reason that the hotter work zone tends to increasingly soften and deform before the cooler regions near the specimen ends do. This, in turn, tends to limit the maximum amount that the specimen can be compressed during each deformation and hence the maximum strain and strain rate that could be imparted to the specimen.
To eliminate these longitudinal gradients, conventional anvil assemblies can be self-heated or separately heated. Unfortunately, owing to the relatively large mass of the anvils compared to the specimen, the heating time of the anvils would be considerably larger than that of the specimen which, in turn, limits a maximum rate at which the specimen could be heated. This, in turn, slows the thermal response of the entire system and is particularly problematic if more than one test temperature is required for a given thermo-mechanical program, i.e., each successive deformation (“hit”) is to be performed at a different specimen temperature. Specifically, this requires that the anvils reach thermal equilibrium at each such successive programmed temperature before each hit occurs. Depending on the temperature excursions involved, the required anvil heating or cooling times could drastically slow the overall response of the system. Modern material production equipment, such as multi-stand rolling mills, often dynamically deforms materials at relatively high-speeds. Hence, any appreciable diminution in the response of the test system, such as those imposed by limits associated with the anvil heating and cooling rates, could severely and adversely limit the production processes that could be accurately simulated by such systems, thus potentially diminishing the attractiveness and cost-efficiencies otherwise attainable through use of such systems.
As increasingly high-speed production processes are currently being employed in industry, a concomitant need has recently arisen with conventional thermo-mechanical material testing systems to impart increasingly higher amounts of strain and strain rates to specimens. Therefore, a need exists in the art for another approach which can be used in such systems for substantially, if not totally, eliminating longitudinal thermal gradients from appearing in self-resistively heated test specimens that are held in anvils (and jaw assemblies). Ideally, such an approach should not appreciably slow the response of the system but still yield isothermal planes in the specimens.
To address this need, the art teaches one approach to substantially eliminate these thermal gradients; namely, by generating sufficient self-resistive heat within each anvil to approximately equal that which would otherwise flow from the specimen into the anvil. This, in turn, would eliminate much, if not substantially all, of any temperature difference otherwise occurring between a top surface of each anvil and the specimen bulk and hence preclude any appreciable longitudinal thermal gradients from appearing in the specimen.
To effectuate this approach, the art teaches that each anvil can be formed of an anvil stack having cylindrically shaped upper and lower members separated by a foil interface having relatively high-resistance compared to the anvil. The sides of the anvil stack are electrically and thermally insulated from its supporting structure by a suitable insulating member, typically a woven ceramic tubing or rigid ceramic sleeve. As a result and during one-half cycle of applied current, self-resistive heating current axially flows up from the support through a base of the anvil stack, through the high-resistive foil interface, through an anvil top and into the specimen end (and in an opposite direction during a next half-cycle of applied current). Since the resistance of the foil interface is relatively high compared to the anvil, passage of the heating current through the foil interface causes it to self-resistively heat with the heat propagating throughout the entire anvil, including the anvil top. The foil interface is typically formed of a stack having a predefined number of graphite disks, with each disk being of an approximate diameter of the anvil and of a given thickness to provide a desired resistance. Unfortunately, the graphite disks, when exposed to the rather high impact forces transmitted to the anvil stack during each hit, are rather compliant and tend to deform and unevenly so, from one disk to the next, from each impact. The result is that the abutting electrical contact amongst the graphite disks as well as the resistance of each disk changes with each hit which, in turn, adversely affects the passage of heating current and hence the heating of the specimen.
Therefore, a need still exists in the art for an approach, for use in a dynamic thermo-mechanical material testing system, that can effectively and substantially, if not totally, eliminate thermal gradients from appearing in a specimen then under test while still permitting isothermal planes to occur in the specimen, but without deforming, to any appreciable extent, if at all, under the high impact force generated during each hit.
Furthermore, a considerable number of conventional, commercially available dynamic material testing systems only provide mechanical deformation of the specimen without any capability of thermally processing of the specimen. These systems basically only compress a specimen held end-to-end between two anvils which are themselves moved by a servo-hydraulic or screw driven actuators to controllably squeeze the specimen. However, to accurately simulate production processes, the specimen under test needs to undergo controlled uniform thermal processing synchronized to the occurrence of the mechanical deformations. As such, these systems need to be modified, by the addition of appropriate apparatus, to possess the capability of providing accurate self-resistive specimen heating that establishes isothermal planes across the specimen under test but without causing appreciable, if any, longitudinal temperature gradients to appear along that specimen. Here too, this apparatus should not deform, to any appreciable extent, if at all, under the high impact force generated during each hit. Hence, a need also exists in the art to provide these capabilities in such conventional mechanical testing systems.
Should these needs be met, then, the attractiveness of physically simulating high-speed production processes through dynamic material testing equipment could very well increase, with advantageously substantial cost savings flowing there from to their users.